A line on your ball.
That's simply a byproduct of allowing you to lift, clean, and replace your ball. So, it's not just on the green. If you have to lift your ball because it interferes with another player, for example, you could replace it in a different orientation. Or if you have to lift it to identify it. Or in the teeing area, you could use an "alignment line."
Ah, JVDB beat me to it.